1. Field of the Invention
Embodiments of the present invention relate to an image forming apparatus in which a latent image formed on the surface of an image bearing member is developed into a toner image by a development device employing a one-component developer developing method for transfer onto a recording medium.
2. Description of the Related Art
Electrophotographic image forming apparatuses generally employ developing methods using either one-component developer or two-component developer. As disclosed, for example, in Japanese Patent Application Publication No. 2008-058667 (JP-2008-058677-A), the one-component developer developing method uses only toner to develop an image, whereas the two-component developer developing method uses developer that includes both toner and carrier. Generally, the one-component developer developing method requires a simpler configuration, and therefore can be used in an easily-maintainable, low-cost developing device.
In the one-component developer developing method, as toner carried on the surface of a toner bearing member passes beneath a layer thickness regulating member as the toner bearing member rotates, the toner is slidably pressed hard against the surface of the toner bearing member and charged by friction (i.e., triboelectrically charged) while it forms a thin layer. Then, as the toner bearing member continues to rotate, the thus-regulated toner is brought to face an image bearing member, where the toner is electrically attracted to an electrostatic latent image formed on the surface of the image bearing member, so that the electrostatic latent image can be developed into a visible toner image.
In the one-component developer developing method, a non-charged surface of the image bearing member passes a development area while an image formation start operation is performed before the start of image formation and/or while an image formation end operation is performed after the image formation. At this time, a very small amount of toner may be attracted to the surface of the image bearing member facing the toner bearing member in the development area from the surface of the toner bearing member. Once this unnecessary attraction of toner occurs, even though the amount of toner per attraction is very small, the accumulated amount of waste toner after repeated attractions becomes too large to ignore. Therefore, it is desired to prevent occurrence of the unnecessary attraction of toner.
Most contemporary image forming apparatuses are designed to uniformly charge the surface of the image bearing member to a predetermined potential having the same polarity as that of the toner used therein, and expose the surface of the image bearing member based on image data so as to reduce the potential at the exposed portion (i.e., a latent image formed portion). By using an electric potential difference between the latent image formed portion and the surface of the toner bearing member, toner on the surface of the toner bearing member is attracted to the latent image formed portion. To prevent the unnecessary attraction of toner in this configuration, it is useful to supply a voltage having the opposite polarity to a development voltage that is output for image formation while the non-charged surface of the latent image bearing member is passing over the development area.
With this method, when the non-charged surface of the latent image bearing member passes the development area, an electric field that can hold toner on the toner bearing member in the development area can reliably be generated, thereby preventing the unnecessary attraction of toner reliably. Further, such a reversing development field can be formed by switching the output polarity of the developer power source that outputs a forward development voltage to the toner bearing member to the reverse development voltage and supplying the reverse development voltage to the toner bearing member, thereby achieving a simple configuration. Accordingly, in comparison to a configuration in which a power source for supplying the reverse development voltage is provided separately from the developer power source, a low-cost configuration can be achieved, which is suitable for the one-component developer developing method that also has a merit of low cost.
As described above, however, since the one-component developer developing method causes the layer thickness regulating member to press toner hard against the surface of the toner bearing member to increase the amount of charged toner, toner may form a film on the surface of the toner bearing member after repeated image formation, which is called “toner filming”. Once toner filming occurs, when the toner on the toner bearing member passes by the layer thickness regulating member, the toner cannot slidably contact the surface of the toner bearing member with friction, and frictional charging of toner cannot be performed reliably. Accordingly, the amount of charged toner to be conveyed to the development area becomes reduced or short, which makes the toner less susceptible to the reversing development field. As a result, a force to attract toner to the toner bearing member is decreased because of the reversing development field, and unnecessary attraction of toner occurs even if the reversing development field is generated.
If a power source having a high capacity capable of outputting a large voltage is used as a developer power source, a sufficiently large reverse development voltage can be output to the toner bearing member in the image formation start operation and the image formation end operation. In this case, a difference in potential between the surface of the latent image bearing member (substantially 0 [V]) and the surface of the toner bearing member in the development area can be large, and a substantially strong reversing development field can be generated in the development area. Therefore, even if the amount of charged toner is not sufficient, the toner bearing member can hold the toner reliably, and therefore, even if toner filming occurs due to age, unnecessary attraction of toner can be prevented.
However, a power source having a high capacity is rather expensive, and therefore it is not preferable to use such a high-capacity power source as a developer power source in an image forming apparatus employing a one-component developer developing method having a merit of low cost. Therefore, it is desired to produce a new method that, even when an inexpensive, low-capacity developer power source is used to output a reverse development voltage to the toner bearing member, a sufficiently strong reversing development field is generated in the development area to hold insufficiently-charged toner to the toner bearing member reliably.